Cement clinker ( Portland cement clinker , short: clinker) is the burnt component of the cement that is responsible for hardening when water is added. The name clinker comes from the similarity to high-fired ( sintered ) bricks ( clinker ). However, cement clinker should not be confused with the brick product .

Clay minerals and lime are mainly used as starting products, mostly in the form of clay and calcareous rocks such as limestone or marl . The raw materials are ground and then heated at around 1,450 ° C until they partially fuse together ( sintering ). The now spherical material is cooled and ground again to obtain cement . Quartz sand and materials containing iron oxide can be added before firing to improve sintering. In order to obtain cement types with certain properties, blast furnace slag , fly ash and gypsum can also be added, for example .

The four phases of clinker essentially arise during sintering:

• Tricalcium Silicate (Alit) C3S
• Dicalcium silicate (belite) C2S
• Tricalcium aluminate C3A
• Tetracalcium aluminate ferrite C4AF

The clinker mainly consists of the following phases :

Clinker phase chemical formula Short formula
Tricalcium silicate ${\ displaystyle \ mathrm {3CaO \ cdot SiO_ {2}}}$ ${\ displaystyle \ mathrm {C_ {3} S}}$ Alite
Dicalcium silicate ${\ displaystyle \ mathrm {2CaO \ cdot SiO_ {2}}}$ ${\ displaystyle \ mathrm {C_ {2} S}}$ Belite
Tetracalcium aluminate ferrite ${\ displaystyle \ mathrm {4CaO \ cdot (Al_ {2} O_ {3}, Fe_ {2} O_ {3})}}$ ${\ displaystyle \ mathrm {C_ {4} (A, F)}}$ Brownmillerite
Tricalcium aluminate ${\ displaystyle \ mathrm {3CaO \ cdot Al_ {2} O_ {3}}}$ ${\ displaystyle \ mathrm {C_ {3} A}}$ Calcium aluminate
free CaO ${\ displaystyle \ mathrm {CaO}}$ - Free lime

The most important phase for hardening is the tricalcium silicate, which has the highest initial hardness.

From a chemical point of view, Portland cement clinker consists of the following main components:

component Average share [% by mass]
${\ displaystyle \ mathrm {CaO}}$ 66.5
${\ displaystyle \ mathrm {SiO_ {2}}}$ 21.5
${\ displaystyle \ mathrm {Al_ {2} O_ {3} + TiO_ {2}}}$ 05.5
${\ displaystyle \ mathrm {Fe_ {2} O_ {3}}}$ 02.5

The main component alite is formed in the presence of a melting phase from around 1250 ° C. In order to achieve a sufficiently complete reaction (residual free lime content below 1.5%), temperatures of around 1450 ° C. are necessary for a period of 5 minutes; there should prevail a predominantly oxidizing atmosphere in the combustion unit and the precursors belite and free lime are homogeneously.